Sounds like mixed COEs.
Seems more likely to happen with soft glass rods vs boro.
If not that, maybe poor quality glass like Devardi or similar. I've heard they can be a nightmare to work with.
Lastly, I would suggest trying to anneal with slower ramps. If your calculations say you can drop at a rate of 8 degrees per minute, maybe slow it down to 5 degrees. That may save the beads but still doesn't address the root problem.
Good luck!
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